Lazy load correlation IDs event sources

[simontabor]

This is a Feature Proposal

Description

• After Feature/refactor correlation ids #57, we will load up all of the event sources (individual files) on cold-start
• Most lambda functions will only have a single event source, so only one of these files needs to be loaded
• As discussed on the PR, we could lazily load the event sources as required.

The implementation could look something like this:

const eventSourceDetector = require('./event-source-detector');
const getEventSource = event => {
  const source = eventSourceDetector(event);
  if (!source) return require('./event-sources/generic');
  return require(`./event-sources/${source}`);
});
// ...
const eventSource = getEventSource(event);
eventSource.captureCorrelationIds(event, context, sampleDebugLogRate);

eventSourceDetector would basically combine the isMatch functions from all the sources in one function, which isn't quite so nice to maintain but would have reasonable cold-start time improvements. require caching is very efficient, so we can rely on that for the lazy-loading.

As this is an optimisation, we need to know whether the change actually has any useful impact on cold-start times before merging any changes.